1. Field of the Invention
The present invention relates to a filter unit and a method of producing a filter unit. Preferably, but not necessarily, the filter unit is an oil/air separator.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
A typical air/oil separator comprises a tubular filter element bonded at each end to a gas-impervious plastic or metal end cap. The filter element maybe formed from the following coaxial components: (i) a tube of filter medium, (ii) a first foraminous support sleeve located on the downstream side of the filter medium tube, and optionally a second foraminous support sleeve located on the upstream side of the filter medium tube, and (iii) a highly porous drainage sleeve located on the downstream side of the first support cylinder. The integrity of, particularly, the bond between the filter medium and the end caps is important for the safe and effective use of the separator.
High efficiency separators that are used in oil lubricated compressors and vacuum pumps can operate at temperatures of well over 80 C and can reduce high inlet oil aerosol concentrations of typically 500 to 2000 mg/m3 to less than 5 mg/m3 outlet. Conventionally, such separators have metal or plastic support sleeves which prevent collapse or rupture of the separator due to high pressure drops. Further, they typically have metal or plastic end caps resin bonded to a filter element, which may utilise a deep bed filter medium containing borosilicate glass micro-fibres. A potting resin, such as a two part epoxy resin, is often used. However, a resin bond, if incorrectly established, can compromise the integrity of the bond between the filter medium and the end cap and result in a complete or partial failure (air bypass and excessive oil carryover) of the separator.
The use of resin systems can provide manufacturing problems. For example, it can be difficult to ensure that the correct volume of resin is dispensed within the end cap. Further, ensuring that the resin mix ratio and/or heat cure cycle is maintained to provide the optimum cure strength can be problematic. Also, two part resins which are dispensed through static mixers can be susceptible to trapped air pockets or bubbles. Additionally, a weak joint can be produced if surface cleanliness and surface texture are not strictly controlled.
A further problem associated with most potting resins is that they are not electrically conductive. Static electricity can build up as gas passes through the filter element, and if this electricity is not safely discharged, sparks can be generated, which in turn may lead to fire or explosion. The problem is particularly acute in the oil-saturated conditions in which separators operate.
Where conductive end caps and support sleeves are used with a non conductive resin, a conductive assembly can still be achieved by forcing the conductive components together and holding them under pressure until the resin sets to complete an electrical circuit. However, poor surface contact in the presence of an electrically insulating potting resin can prevent or inhibit static electricity on the filter element from discharging via the end caps.
One proposal, described e.g. in GB A 2300367, is to use a conductive resin in the construction of the filter to conduct static electricity away.